PDCD1 and plasma cell myeloma: An in vivo murine myeloma model demonstrated that the expression levels of immune checkpoint receptors such as programmed cell death protein 1 (PD-1), T-cell immunoglobulin and ITIM domains (TIGIT), lymphocyte activation gene-3 (LAG-3), and T-cell immunoglobulin mucin-3 (Tim-3) were increased on CD8+ T cells in bone marrow in MM-relapsed mice compared with those in control mice and MM-controlled mice [82].